When Chinese hamster cells (line CHO) are cultured in a medium which is dificient in isoleucine, essentially all cells accumulate and are arrested in the G1 phase of their life cycle. Upon addition of isoleucine, the G1-cells resume cell cycle traverse and initiate DNA synthesis and cease division in synchrony. We propose to investigate the role of isoleucine in the initiation of DNA synthesis by (1) attempting to detect, isolate, and characterize regulatory macromolecules whose synthesis requires isoleucine, (2) determining the effect of isoleucine-deprivation on mitochondrial DNA synthesis, and (3) using the isoleucine method of producing G1 cells to evaluate structural analogs of isoleucine or other drugs for efficacy in preventing initiation of DNA synthesis.